Dry Milk Powder Reconstitution Process - The Basic Science Explained
Reconstitution versus 'dissolving'
For this discussion, the term 'reconstitution' refers to the process of recreating, as complete as possible, the original liquid state of the milk before it was dried into a powdered form. In that process, not everything will be completely 'dissolved', or go into a complete aqueous solution when mixed with water.
Take for example that whole fresh milk is actually a mix of the following three components:
1.) Fat globules of different sizes suspended in an aqueous medium as a natural emulsion (such as a vinaigrette dressing, since oil (fat) is not soluble in water);
2.) Proteins in a suspended colloidal state (not unlike a slightly runny Jello); and
3.) Carbohydrates in true solution (such as a transparent and clear dissolved mix of Gatorade).
The main purpose of making of dry milk powder is to convert this liquid, perishable raw material into a product that can be easily transported and stored without spoilage and substantial loss of quality when properly stored. The challenge of the rehabilitator is to reverse this dehydration process by transforming the powder back to a liquid state, aka reconstitution.
The following is a very simplified discussion of the various steps that occur during the process of reconstituting a powdered substance (such as powdered milk) into a liquid (such as water). As will be discussed, each of these various steps, while at times occurring coincident and simultaneously, must each occur in order to achieve complete dissolution. A more in-depth and complete examination of this process is available through the references listed below.
The physicochemical steps of the reconstitution process
Powders that disperse in both hot and cold water with a minimum of stirring and without the formation of lumps or undissolved sediments are referred to as 'instant' powders. As shown at right (adapted from Laurent, 2009), the reconstitution of instant powders can be described through several steps. There are many studies and researchers describing these steps as to their order, degree of overlap, and using a variety of terminology. In general, they are primarily described as follows:
1.) The wetting of the milk particle (agglomerate) followed by the penetration of the water into the pore system (see below) of the particle due to capillary forces;
2.) The sinking or immersion of the agglomerate into the liquid;
3. The disintegration of the agglomerates as they break apart into smaller particles followed by the dispersion of the particles within and throughout the liquid volume (water); and finally
4.) The dissolution of soluble primary particles. This also includes dissolution of emulsified fats and the colloidal dispersion of proteins. Some undissolved small aggregates may still remain at this point, but should be few in number.
Since in the actual reconstitution processes these steps can not be clearly isolated from each other, they do not happen sequentially, but rather many or all can occur simultaneously depending on the nature of the powder and the methods used to reconstitute the powder.
Factors that can influence the reconstitution process
While there are many factors that are beyond the scope of this discussion, here are five that rehabilitators can consider and influence.
1.) Product formulation. For purposes of this discussion, this relates to the type of drying process (from liquid to powdered milk) that determines dry particle size and its properties such as porosity. Simply stated, porosity is the property of the particle's affinity to absorb water. The spray drying process used by PetAg® increases the size of the dry particles by introducing air into the particles as well as 'deforming' the surface area to allow for more contact with the water. These exterior ridges and interior hollows or pockets of air allow for an easier penetration of water into the particle through normal capillary action. A scanning electron microscope (SEM) image of these types of particles is shown below (file image; not a PetAg® product):
Fox Valley produces a dry powdered milk product with a much smaller particle size and thus has less relative porosity. The porosity of any powder is important in that it can help or hinder performance during the wetting process, which is the initial step in reconstitution.
2.) Wetting. Both PetAg® and Fox Valley provide mixing directions on the product packaging that specifies to add the powder to the water. This is intended to allow for the 'wetting' of the powder as the first step in the reconstitution process, as the powder begins to absorb water. In WildAgain's tests, the powder is placed on top of the liquid for 5 minutes prior to stirring or mixing. Results of these tests indicate PetAg® products begin to wet almost immediately while the Fox Valley products show little apparent wetting. This difference is likely due to the porosity of the respective dry milk particles.
3. Sinking. As the PetAg® dry particles begin to quickly absorb water, they take on added weight, break the surface tension of the water, and tend to sink 50-90% over a 5 minute wetting/sinking time interval. Since the Fox Valley dry products demonstrate very little sign of wetting, there is little or no sinking that occurs without mechanical intervention.
Wetting and sinking test
Samples of powdered milk (the four products shown below) were placed on water at 100°F and at 160°F at a ratio of 1 part powder with 2 parts water. The degree of wetting was observed over a three minute period, photographing the result. Observations were also taken at 15 minutes.
The results observed indicated the following:
1.) None of the powders conformed to the 5-second wetting/sinking requirement to be considered 'instant-mix' products.
2.) All of the products tested showed relatively better wetting/sinking performance at the higher water temperature.
3.) After 15 minutes, only a small amount of additional wetting /sinking was noted.
4.) Based upon on degree of wetting/sinking exhibited by all products, considerable additional stirring was required to advance the wetting process.
Fox Valley 32/40. Minimal wetting at either temperature, though marginally improved at the higher temperature.
Fox Valley 20/50. Minimal wetting at either temperature, though marginally improved at the higher temperature for wetting and some sinking.
Zoologic 33/40. Similar wetting at either temperature, though sinking of large clumps beginning at the higher temperature.
Esbilac (33/40). Similar wetting at either temperature, though sinking of large clumps beginning at the higher temperature.
4. Dispersion. The application of energy is required in order to accomplish the disintegration of the of the agglomerated (clustered) dry milk particles. The energy requirements depend on the size, shape and physical state of the chemical bonds that form the agglomerate clusters and particles. This energy can be applied in two ways.
First, mechanical mixing is required by either hand stirring, whisking, blending, shaking (in a closed container) or other forms of mixing. With the poor wetting/sinking properties shown above, large unwetted agglomerates (shown right) must be broken up and dispersed using mechanical means for several minutes.
Second, using warm or hotter water temperatures can also promote more complete dispersion and dissolution. However studies have shown that a more complete mechanical mixing has a higher degree of impact on faster and more complete dissolution of the agglomerated particles.
While both manufacturers indicate the products can be used as an "instant mix," WildAgain's tests indicate more complete dissolution to smaller (wet) particle size with up to 90% improvement if allowed to rest, and further complete the dissolution process, for up to 8 hours in the refrigerator. See more here on the tests and results.
5. Dissolution. WildAgain's tests confirmed a range from good to poor dissolution (solubility) for the various products following the IDF standard insolubility index test procedure (rehydrating with 110°F water for only about 30 minutes). Scientific studies and papers indicate that the casein molecules and larger casein clusters require the longest time for complete dispersal and dissolution, and will measurably benefit from longer reconstitution time and hotter water temperatures. This is especially critical for rehabilitators to achieve a more complete dissolution of the casein-rich milk powders.
As discussed above, reconstituting a powdered milk product into a completely dissolved and reconstituted state is more complex and involved that simply adding the powder to the liquid and mixing for immediate use. While some products may perform better (faster, easier, and completely) such as human grade of instant coffee creamer, a more complex powder such as those used by rehabilitators requires far more attention to the steps described above to achieve an optimal result. Follow this link for a more thorough discussion on preparing powdered milk replacers for use with wildlife.
References (not intended as an exhaustive list)
Anema, S. G., D. N. Pinder, R. J. Hunter, and Y. Hemar. Effects of storage temperature on the solubility of milk protein concentrate (MPC85). Food Hydrocolloids (2006) 20:386-393.
Baldwin, Alan J., Fonterra Research Centre, Palmerston, NZ. Insolubility of milk powder products- a minireview. Dairy Science Technology (2010) 90:169-179.
Boiarkina, I., N. Depree, W. Yu, D. I. Wilson, and B. R. Young. Rapid particle size measurements used as a proxy to control instant whole milk powder dispersibility. Dairy Science and Technology (2017) 96:777-786.
Forny, Laurent and Stefan Palzer (Food Science and Technology Department, Nestlé Research Centre, Vers-Chez-Les-Blanc, CH-1000 Lausanne, Switzerland). Wetting, disintegration and dissolution of agglomerated water soluble powders. Conference Paper · June 2009
Ilari, Jean-Luc and Laila Mekkaoui. Physical properties of constitutive size classes of spray-dried skim milk powder and their mixtures. Le Lait, INRA Editions, 2005, 85 (4-5), pp.279-294. hal-00895603
International Diary Federation. In determination of insolubility index, Standard 129A. Brussels, IDF (1988)
Pugliese, Alessandro, Giovanni Cabassi, Emma Chiavaro, Maria Paciulli, Eleonora Carini, and Germano Mucchetti. Physical characterization of whole and skim dried milk powders. Journal of Food Science Technology (2017) 54(11):3433-3442.
Sharma, Anup, Atanu H. Jana, and Rupesh Shrikant Chavan. Functionality of milk powders and milk-based powders for end-use applications - a review. Comprehensive Reviews in Food Science and Food Safety (2012) 11:518-528.